Shales black/organic-rich

The Bossier shale is not only the basal formation for much of the Cotton VaUey Group, but is also interbedded with isolated sands throughout most of eastern Texas and northwestern Louisiana. These shales are typically black, organic-rich, calcareous, fossiliferous, marine deposits which are the primary source rock for much of the entire Upper Jurassic and Lower Cretaceous vertical section. Thinner shale intervals in the western part of the basin thicken to the east correlative to general depositional patterns in the East Texas Basin. Source rock quality is generally poorer in the western and northwestern parts of the basin, but improves significantly easterly towards the basin centre. From west to east, the near-shore sand and low-stand fan deposits are usually completely encased in the marine Bossier shale. 

Although the differences in physical and chemical properties of selenium and sulfur tend to separate these elements in weathering and erosion, their similar functions in biological reactions tend to unite them in organic-rich materials. Selenium is found often manyfold its crustal abundance in black organic-rich shales, in coal, and, to a lesser extent, in petroleum. 

As is the case for Sr/ Sr, there is a strong increase of Os/ Os towards more radiogenic values over the past 14 My. As stated above, this has been linked to the rise of the Himalayas, but recent analyses of Himalayan river waters for Os isotope compositions do not support this view. A more likely possibility is the weathering of ancient crystalline terranes exposed by physical erosion, or the weathering of black shales. These organic-rich sediments have a high Re/Os ratio. Therefore old black shales have the potential to supply Os with a very high Os/ Os ratio to sea water. 

The Fayetteville Shale Play is located in the Arkoma Basin of northern Arkansas and eastern Oklahoma and ranges in depth from 1000 to 7000 feet (Table 5.3). The Fayetteville Shale is a black, organic-rich, Mississippian-age shale bounded by limestone (Pitkin Limestone) above and sandstone (Batesville Sandstone) below (Figure 5.3). 

Black organic-rich shales also have a tendency to concentrate Re over Os, albeit not to the same extent as molybdenite, so isochron rather than single sample dating needs to be performed. The ages obtained are interpreted as the depositional age of the sediments and the initial Os/ Os ratio as that of seawater at the time of deposition [116]. 

The behavior of uranium varies considerably across samples studied. Palmer and Edmond (1993) have reported increasing trends of uranium concentration with river alkahnity in the Orinoco, Amazon and Ganga river basins, showing the importance of limestone and black shale dissolution for the control of uranium concentrations in river waters. The association of uranium and major soluble elements is also reported by Elbaz-Poulichet et al. (1999) for the upper Amazonian basins of Bohvia. However, under the organic-rich conditions of Scandinavian rivers (Porcelli et al., 1997) or African rivers (Viers et al., 1997), uranium concentrations can be decoupled from those of major elements, due to the existence of a colloidal fraction of uranium. 

In 1981, the Ontario Geological Survey began a program to assess the resource potential of the Whitby Formation and the other black shales of Ontario. Prior to this study, information on Ontario s black shales has been very sparse due to poor or non-existent exposure and limited subsurface information. Existing data suggested three units were sufficiently organic-rich to warrant further study—the Ordovician Whitby and the Devonian Kettle Point and Marcellus Formation (JL). ... 

The Antrim Shale play is a brown to black, pyritic, and organic-rich shale located in the upper portion of the lower peninsula of Michigan within the Michigan Basin. This Late Devonian-age shale is bounded by shale (Bedford Shale) above and by limestone (Squaw Bay Limestone) below and occurs at... 

Black shales that are formed in an anoxic environment such as the Black Sea have a Mo isotope composition nearly identical to ocean water (Barling et al. 2001 Arnold et al. 2004 Nagler et al. 2005). Organic carbon rich sediments formed in suboxic environments have variable Mo/ Mo ratios intermediate between those of ocean water and oxic sediments (Siebert et al. 2003). Thus Mo isotope values in ancient black shales can be used as a paleo-oceanographic proxy of the oxidation state of the ocean, as for example has been discussed by Arnold et al. (2004) for the Proterozoic. Figure 2.25 summarizes natural Mo isotope variations. 

Moldowan et al (5) investigated a sediment core, spanning a depth range of 5m, of Lower Toarcian shales from W. Germany specifically at a transition zone from a rather oxidized, shallow-marine, marly sediment to an organic matter-rich, black shale. Variations in distributions of isoprenoid hydrocarbons, steranes and monoaromatic steroids were observed and were related to variations in oxidation/reduction conditions during and shortly after sedimentation. 

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